A phenomenon of superconduction is the most specific phenomenon among electro-magnetic characteristics manifested in substances, and development in various field of applications is greatly expected for the utilization of a perfect conductivity, a perfect diamagnetic property and quantizing of magnetic flux.
Among electronic devices utilizing superconduction phenomena are included a high speed switch, a high sensitive detecting element, a high sensitive fluxmeter and many other electronic devices.
As a superconductor which has been widely used in conventional superconductor devices, Nb.sub.3 Ge thin film formed on a substrate by using plasma sputtering technique may be mentioned. However, the critical temperature of Nb.sub.3 Ge thin film is at most 23.degree. K. Therefore, it can be used only within temperatures of liquid helium When liquid helium is used, however, cost for cooling and technological burden necessary to install a liquefying and cooling equipment of helium are increased. In addition, resource for helium is extremely limited. These problems hindered use of liquid helium in industrial field as well as for public welfare.
Various investigations have been made for obtaining superconductors having high critical temperatures. Remarkable progress has been made in recent investigations on oxide superconductor thin films. As a consequence, the critical temperature of superconductors becomes 77.degree. K. or higher. Thus, the superconductors can operate by using inexpensive liquid nitrogen as cooling medium.
In conventional methods, such oxide superconductor films have been formed on a MgO monocrystal substrate or a SrTiO.sub.3 monocrystal substrate heated to a high temperature by using sputtering or vapor deposition technique.
Among other monocrystals used as substrates, attentions have been paid to sapphire, YSZ, silicon, gallium arsenide, LIN O.sub.3, GGG, LaGaO.sub.3, LaALO.sub.3, etc.
In a conventional method of manufacturing superconductor thin films on a MgO monocrystalline substrate or a SrTiO.sub.2 monocrystalline substrate, it is impossible to stably increase the superconduction critical current (Jc). Furthermore there is a problem that the superconduction critical temperature (Tc) is not stable.
To form excellent epitaxial films, the substrate material is required to satisfy the following conditions
(a) To have excellent lattice matching with a thin film crystal. PA0 (b) No degradation of the film quality due to mutual diffusion at the time of epitaxially growing the film. PA0 (c) To have a high melting temperature of at least higher than 1000.degree. C., since the substrate material is heated to a high temperature. PA0 (d) Monocrystals having excellent crystallization is available. PA0 (e) To have electric insulating property.
As oxide superconductors having high critical temperatures, many oxides have been reported such as oxide thin films of LnBa.sub.2 Cu.sub.3 O.sub.7-x (x=0-1, Ln: Yb, Er,Y,H.sub.0, Gd, Eu, Dy) B.sub.i -Sr-Ca-Cu-O and oxide thin films of Tl-Ba-Ca-Cu-O.
The lattice constants a and b of these oxides are all included in a range of 3.76-3.92 .ANG.. By rotating the coordinate system by 45 degrees, .sqroot.2.multidot.a and .sqroot.2.multidot.b can also be deemed as fundamental lattice, in which the lattice constants a and b are expressed by 5.32-5.54 .ANG..
For magnesium oxide (MgO) presently used widely as a substrate material, a=4.203 .ANG. and the difference in the lattice constants reaches 7-11%, so that it has been extremely difficult to obtain excellent epitaxially grown films. This problem has also occurred for sapphire YSZ, silicon, gallium arsenide, LiNbO.sub.3 and GGG.
When compared with MgO, SrTiO.sub.3 has a small difference of 0.4-4% in the lattice constant from the oxide superconductor thin film as well as an excellent lattice matching property. At present, however, SrTiO.sub.3 can be prepared only by Bernoulli method, and its crystal property is very poor so that only crystals having an etch pit density of larger than 10.sup.5 /cm .sup.2 can be obtained. Accordingly, it is difficult to form an excellent epitaxially grown film on such substrate having poor crystal quality. Furthermore, it has been impossible to find a substrate of large size.
LaGaO.sub.3 monocrystal has a lattice constants of a=5.496 .ANG. and b=5.554 .ANG.. This would ensure a good lattice matching with an oxide superconductor. However, phase transition occurs at about 150.degree. C., so that there is a problem of twinning in the crystal. Accordingly, when actually using LaGaO.sub.3 monocrystal as a substrate of a superconductor film, it is necessary to remove twin crystals.
LaAlO.sub.3 monocrystal has lattice constants a=b=3.788 .ANG. and a good lattice matching with an oxide superconductor can be expected. However, since the melting point of the LaALO.sub.3 monocrystal is very high of about 2100.degree. C., it is extremely difficult to manufacture such monocrystal. In addition, such monocrystal likely to contains twin crystals.
As described above, the conventional materials which have been used as a substrate for forming a superconductive film do not satisfy the above-described conditions such as good lattice matching property with the superconductive film and easy availability. Consequently it has been difficult to manufacture stable superconductor devices.
As a result of various experiments, the inventors have proposed a monocrystal substrate material capable of forming an epitaxially grown superconductor film, that is, a strontium lanthanum gallium oxide monocrystal substrate having K.sub.2 NiF.sub.4 type crystal structure as shown below. The inventors have presented a method of forming an oxide superconductive film on this substrate with epitaxial growing method (Japanese Patent Application No. 63-323739). EQU Sr.sub.1-x La.sub.1-y Ga-30.sub.4-w
(-0.05&lt;x&lt;0.05, -0.05&lt;y&lt;0.05; -0.05&lt;z&lt;6.05, -0.2&lt;w&lt;0.2)
The difference in the lattice constants of SrLaGaO.sub.4 monocrystal and an oxide superconductive film is extremely small, that is -1.6-2.6%. Their crystal structures are extremely similar to each other. Furthermore, the lattice matching between the SrLaGaO.sub.4 monocrystal and an oxide superconductive film is excellent. In other words, both SrLaGaO.sub.4 monocrystal and the oxide superconductive film satisfy every required conditions described above. However, in the manufacture of SrLaGaO.sub.4 monocrystal, such problems as cracks, subgrains, cells, etc. are liable to occur so that it has been impossible to manufacture monocrystals at high efficiencies.